'81 MKI, all 4 brakes locked

[spamalope]

So, the brake system is not bleeding off back pressure after stopped a few times and all four brakes lock up and stay locked up with a hard pedal. The car sat for about 8-9 months, likely a brake master cyl or the prop valve? Where can one get a new prop valve? Thanks, Frank

[Supradad]

Let me check my shed and I'll get you some pix.
I stripped down an 81 Mk1 a couple years ago and I saved alot.
I know those parts were in great condition and not the original and old stuff.

[spamalope]

good to know that I might have a line on parts but does the MC and/or Prop valve sound about right? Thanks, frank

[DRIFTINGmy85]

I don't understand how the proportioning valve would make all 4 wheels stay locked up. I'm not that experienced with brakes and how they go wrong, though I've learned, but you sound sure that it's the proportioning valve?

[spamalope]

well I'm not %100 sure and that's why I'm asking cause I could understand it being either the prop valve or the MC no bledding the pressure back. Anyone dealt with this issue before?

[DRIFTINGmy85]

I'll ask the brakes teacher at my school. Let's see what he has to say about it.

[DRIFTINGmy85]

First, what condition is the fluid in? When was the last time the system was flushed? Water could have accumulated in the system causing rust and corrosion in the system. By mistake, oil may have made it into the system. Check the condition of the fluid. Is it clear, black, or brown? Take a drop of your brake fluid from the master cylinder and drop it into water. How fast does it absorb into the water? Or does it stay separated? The faster and more complete it dissolves/absorbs into the water indicated better fluid. Less water content is better because brake fluid is hydro-scopic, absorbs water easily.
A brake fluid flush is easy and usually needed.

Try these:
The square cut seals keep the brake system from leaking at the brake piston and retract the brake piston too.

The slider pins may no longer be lubricated and froze themselves stuck needing to be broke loose, taken apart and lubricated with grease.

Stick a large screwdriver in between the brake pads and the rotor to retract the piston into the caliper. This should break loose the pistons and caliper assembly.

It could also be clogged compensation ports on the master cylinder. Try breaking loose the slider pins and piston and see if that's the problem.